I was hoping to find this explanation after trying to fit the 'y' sample test you mentioned with the rest.

Re-posted as an issue.

BTW there's sample test with 'y' which should give a hint, but I agree that 'y' being/not being a vowel can come as surprise for people using different languages.

Since notion of a vowel is ambiguous, description should explicitly state what is considered vowel by this kata. Otherwise it raises doubts and misunderstandings (see @mpotoczny's post below).

It would be helpful to mention in description that 'y' is not in vowels. I spend much time trying to solve this kata and using vowels "aeiouy". Then I figured out that solution works without 'y', so there was not issue with my code but rather with not precise description.

Clean,solid solution Mati.
Cheers!